The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, a hybrid electric vehicle is a vehicle driven by using, as driving sources, an internal combustion engine using fossil fuel and a motor using electric energy. The hybrid electric vehicle is an eco-friendly vehicle in which the energy of the fossil fuel and the electric energy are used together to drive the vehicle, thereby reducing exhaust gas and improving fuel efficiency.
A transmission mounted electric device (TMED) type in which a driving motor is disposed at a transmission side is well known as the powertrain type of the hybrid electric vehicle. In a typical TMED type hybrid electric vehicle, an engine clutch is interposed between an engine and a driving motor as driving sources, and a transmission is disposed at an output side of the driving motor, so that power of the engine and the driving motor are transmitted to front wheels as driving wheels through the transmission in a coupling state of the engine clutch.
In the TMED type hybrid electric vehicle, the engine and the driving motor are disposed together at the side of the front wheels as the driving wheels, so that only the driving torque of the front wheels is controlled by overlaying torques of the engine and the driving motor prior to the transmission and transmitting the overlayed torque through the transmission.
Alternatively, an electric 4 wheel drive (E-4WD) system in which a propeller shaft that is a mechanical power device is removed and a driving motor is applied to rear wheels in order to improve the efficiency of a vehicle is disclosed in U.S. Pat. No. 7,517,298.
In a hybrid electric vehicle to which the E-4WD system is applied, independent driving means are applied to front and rear wheels, respectively. In this case, an internal combustion engine (ICE) may be used as the driving means of the front wheels, and a driving motor may be used as the driving means of the rear wheels. The driving means may be driven independently or together according to a driving environmental condition.
In a typical E-4WD hybrid electric vehicle, the vehicle is basically driven in a 2 wheel drive (2WD) mode in which the vehicle is driven by using only any one of front and rear wheels as a driving wheel. When driving power is insufficient, the vehicle is driven in a 4WD mode in which the vehicle is driven by using the other wheel in an idle state as the driving wheel.
In the E-4WD hybrid electric vehicle, the drive mode is changed depending on a driving situation so as to improve fuel efficiency. For example, when a slip occurs due to sudden starting, slipper road, sandy soil, steep hill, muddy path, or the like, and therefore, high driving power is required, the vehicle is driven in the 4WD mode. In general high-speed driving, the vehicle is driven in the 2WD mode.
When the vehicle is driven in the 4WD mode, it is advantageous in terms of posture control or steering ability of the vehicle, climbing performance, and escape of rough road. However, in the 4WD mode, fuel efficiency is deteriorated as compared with that in the 2WD mode. When the vehicle is driven in the 2WD mode, driving torque is small, and there is no excess in terms of steering stability. However, in the case of a rainy road, snowy road or frozen road, the road surface friction coefficient of the road decreases, and hence the friction limit of tires also decreases. Therefore, it is difficult to control the posture of the vehicle.
Meanwhile, when the E-4WD hybrid electric vehicle is driven in the 4WD mode, the driving motor at the rear wheel side assists driving power required in the vehicle as described above. Therefore, it is required to efficiently distribute output torque of the engine and output torque of the driving motor.
Conventionally, a fixed distribution method of distributing power at a fixed ratio to the engine at the front wheel side and the driving motor at the rear wheel side was used as the method of distributing power to the engine at the front wheel side and the driving motor at the rear wheel side. However, in the fixed distribution method, the electric energy of a battery cannot be efficiently used, and the energy management of the battery is not efficiently performed.
In addition, studies on control of front/rear wheel torque, control of efficient front and rear wheel distribution, and the like for the purpose of vehicle stability according to a difference in wheel speed between front and rear wheels, a driver's intention of sudden acceleration, or a handling situation of a vehicle are unsatisfactory.